scsi: scsi_debug: Use struct t10_pi_tuple instead of struct sd_dif_tuple

And remove the declaration of the latter in sd.h as scsi_debug was the
only user.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Bart Van Assche <bart.vanassche@sandisk.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>